I am aware of only two places that moderators can customize their Help Centers routinely.
I'll use the GIS Help Center as an example because being one of its moderators allows me to screenshot its edit buttons.
The best known is the On-topic page which bears the title What topics can I ask about here? The title is the only thing that we cannot change on this page. If I scroll to its bottom there is an edit button that lets me and the other GIS moderators edit its content. We do not do that without consulting the GIS Meta community first to ensure there is consensus for what we intend to do. My recollection is that when we change the content a second moderator has to approve it. A history of those changes is kept.
The other place we can edit, which I only noticed very recently, and we have not edited it yet, is on the front page of the Help Center:
Anything you see above Find out more about... has been edited in by a site's moderators.